A couple of weeks ago the world heard about the most seriously funded (and perhaps the most serious) effort yet for starting us on the pathway to interstellar travel.

The Breakthrough Starshot project won't take people to another star, or even take a conventional robotic explorer. Instead the goal is to propel nano-spacecraft, 'Starchips', to the Alpha Centauri system, using the pressure of light on four-meter sails of reflective material. These ultra-lightweight vehicles would end up shooting through that distant system (some 4.3 light years, or 26 trillion miles away) with a velocity of some 20% of the speed of light - a mind-boggling 134 million miles per hour. Such a blistering pace would be achieved in the first couple of minutes of 60,000 g acceleration from a near-Earth starting point.

It is, to say the least, pretty audacious.

So does it have a chance of working?

As someone not currently involved with the project I've nonetheless spent an inordinate amount of time these past days digesting and fretting over the details. It's hard not to get caught up in the excitement, the sense that maybe, just maybe, there will be a critical mass of work applied to this idea to finally get our species further along the path to the rest of the universe. The possibility is hugely compelling.

There is also the sense that even if the technical hurdles prove to be insurmountable at this stage of our civilization, the quest to get past them will itself be a boon for space science and technological development (a fact clearly not lost on the Breakthrough organization).

But, some of these hurdles are truly headache inducing.

Take for example the proposed requirements of a massive grid of phased lasers that would fire from the surface of the Earth to accelerate the tiny lightsails. The estimated power use is about 100 GW for an Alpha Centauri trip (although only for a few minutes). That's a big number. A run-of-the-mill single reactor nuclear power station typically produces around 1 to 1.5 GW. So if you wanted a continuous feed of 100 GW, well that's 100 nuclear power plants. Of course you'd likely deploy some kind of energy storage - a reservoir you could charge up with far fewer primary generators. But on this scale that means entirely new types of super-capacitors or chemical energy stores.

Then there's the inherent inefficiency of carrying that power to the laser array. Ever felt the cable to your electric kettle get warm? That's what happens when you draw power through even a good conductor. Now imagine pulling about 100 million kettles' worth of power, things are going to get pretty warm pretty fast.

Firing lasers through the Earth's atmosphere and retaining coherence and controlling the spread (dispersion) of the beams has its own challenges. But the atmosphere is also never 'clean', it's full of dust, bacteria, and other particulates. When faced with a barrage of photons on this scale it's not entirely clear what's going to happen - that's a lot of stuff getting vaporized along the way. Or worse, scattering some of the power back.

The list goes on to include even more intriguing, but worrisome issues.

The proposed lightsails must be highly reflective to avoid being vaporized by the laser light that's accelerating them. A reflectivity of 99.999% has been quoted (which still involves the sail getting heated by many tens of kilowatts of absorbed power). Except, this figure seems to assume 'ordinary' reflection of light. If we cast our eyes back to Maxwell's equations we know that reflection is due to the response of charges (e.g. electrons) in the mirror material to the incident electromagnetic field. But it's not entirely clear (at least to me) what happens when the photons (fields) are coming as thick as in the Starshot proposal. Is this a non-linear optical regime? It may be that we'd have some very serious photonics to understand.

There are also quite well known questions and challenges about how you keep a lightsail 'balanced' on a beam of radiation. Just like a conventional sail, there is an inherent instability that wants to push the sail off the principal axis of pressure.

And space is hardly benign. How do you build a nano-spacecraft that can deal with the cosmic radiation environment of both near Earth and interstellar space? Particle radiation is nasty stuff, and tiny chip-like instruments can't carry much shielding or redundancy. And traveling at 20% of the speed of light brings a risk that even a molecular collision could damage some vital part.

How do you even communicate with such a tiny ship? The idea of using low-power lasers has been mentioned, or creating a type of interstellar communication infrastructure by exploiting the gravitational lensing power of our Sun. Our star, like any mass, curves the path of light. But the Sun is also a big opaque disk in the sky, so you have to get at least 500-600 astronomical units away for lensed photons from distant sources on the opposite side to make it past the edge of the disk to a focus at your location.

I like this idea, but it requires a significant (and well-powered) instrument placed at a distance some 4 to 5 times further than anything we've yet put into space. And it's got to maintain alignments against the relative motions of our Sun and (for example) the Alpha Centauri system.

So, can Starshot make us an interstellar species?

Perhaps. And despite all of these questions, and many more, what is clear is that Starshot will bring a set of fresh ideas and invigorating challenges to the scientific community. Even if the full mission to Alpha Centauri remains a goal for a distant future, I'd place money on the possibility of the project resulting in a revolution in our exploration of the solar system.

A scaled-down version of all that is being considered - lower power laser propulsion, somewhat more massive spacecraft with larger and less reflective sails, improved low-power communication - could open up our local territory. If a trip to Pluto took a few months instead of ten years, or a probe's visit to Mars was a week or two away, that would also be a future to look forward to.

The views expressed are those of the author(s) and are not necessarily those of Scientific American.

ABOUT THE AUTHOR(S)

Caleb A. Scharf

Dr. Caleb A. Scharf is Director of Astrobiology at Columbia University,and has an international reputation as a research astrophysicist, and asa lecturer to college and public audiences. The UK's Guardian newspaperhas listed his blog Life, Unbounded, as one of their "hottest scienceblogs," while an editor at Seed Magazine called it "phenomenal.Informed, fresh, and thoughtful." Scharf is author and co-author of morethan 100 scientific research articles in astronomy and astrophysics. Hiswork has been featured in publications such as New Scientist, ScientificAmerican, Science News, Cosmos Magazine, Physics Today, and NationalGeographic, as well as online at sites like Space.com and Physorg.com.His textbook for undergraduate and graduate students, Extrasolar Planetsand Astrobiology, won the 2012 Chambliss Prize of the AAS. Hisarticles and reviews have appeared in such prestigious publications asScience, Nature, The Astrophysical Journal, and Monthly Notices of theRoyal Astronomical Society.Dr. Scharf is a regular keynote speaker at academic meetings, such asfor the American Physical Society, museums, and both public and privatevenues, including the American Museum of Natural History, the RubinMuseum of Art in New York. He has been a guest on Krulwich on Science atNPR, William Shatner's "Weird or What?" and has served as a consultantto editors and producers at National Geographic Magazine, The ScienceChannel, The Discovery Channel, and The New York Times.

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